This invention relates generally to sensor systems for detecting analytes in fluids and, more particularly, to sensor systems of this kind that incorporate sensors having electrical resistances that vary according to the presence and concentration of analytes, and to methods of using such sensor systems.
There is considerable interest in developing sensors that act as analogs of the mammalian olfactory system [Lundstrom et al. Nature 352:47-50 (1991); Shurmer and Gardner, Sens. Act. B 8:1-11 (1992)]. This system is thought to utilize probabilistic repertoires of many different receptors to recognize a single odorant [Reed, Neuron 8:205-209 (1992); Lancet and Ben-Airie, Curr. Biol. 3:668-674 (1993)]. In such a configuration, the burden of recognition is not on highly specific receptors, as in the traditional "lock-and-key" molecular recognition approach to chemical sensing, but lies instead on the distributed pattern processing of the olfactory bulb and the brain [Kauer, TINS 14:79-85 (1991); DeVries and Baylor, Cell 10(S):139-149 (1993)].
Prior attempts to produce a broadly responsive sensor array have exploited heated metal oxide thin film resistors [Gardner et al., Sens. Act. B 4:117-121 (1991); Gardner et al., Sens. Act. B 6:71-75 (1991); Corcoran et al., Sens. Act. B 15:32-37 (1993)], polymer sorption layers on the surfaces of acoustic wave resonators [Grate and Abraham, Sens. Act. B 3:85-111 (1991); Grate et al. Anal. Chem. 65:1868-1881 (1993)], arrays of electrochemical detectors [Stetter et al., Anal. Chem. 58:860-866 (1986); Stetter et al., Sens. Act. B 1:43-47 (1990); Stetter et al., Anal. Chem. Acta 284:1-11 (1993)], or conductive polymers [Pearce et al., Analyst 118:371-377 (1993); Shurmer et al., Sens. Act. B 4:29-33 (1991)]. Arrays of metal oxide thin film resistors, typically based on tin oxide (SnO.sub.2) films that have been coated with various catalysts, yield distinct, diagnostic responses for several vapors [Gardner et al., Sens. Act. B 4:117-121 (1991); Gardner et al., Sens. Act. B 6:71-75 (1991); Corcoran et al., Sens. Act. B 15:32-37 (1993)]. However, due to the lack of understanding of catalyst function, SnO.sub.2 arrays do not allow deliberate chemical control of the response of elements in the arrays nor reproducibility of response from array to array.
Metal oxide sensors such as SnO.sub.2 films that have been coated with various catalysts are used primarily to detect gas leaks, e.g., carbon monoxide (CO). Gas leak detection is a specific chemical sensing application in which the existing technology is well-established. The current technology in this area is problematic in that metal oxide sensors work only at high power, i.e., they must be plugged into a conventional ac power source. This is because the metal oxide element must be heated for the chemical detection to work. The heated element literally bums methane, propane, CO, etc., to carbon dioxide. Oxygen for the combustion reaction is actually supplied by the metal oxide itself, and it is the absence of oxygen that sets off an electrical current in the metal oxide element thereby detecting the presence of the gas. Different gases are detected by metal oxide sensors based upon the addition of certain catalysts.
Existing metal oxide sensors are single-channel devices that collect information from a single sensor and that are thus designed to trigger only when the gas, e.g., CO, is present. However, the devices can be prone to false alarms, and in many cases consumers disable the devices because of this annoyance. A sensor is needed with a lower false alarm rate to overcome the problematic user error. This can be accomplished by adding an additional sensor to CO detectors, which provides information that the odor is not CO, thereby lowering the occurrence of false alarms and providing significant product improvement.
Also, profiling a chemical environment, rather than simply indicating the presence or absence of a gas leak, has far more utility in many applications, e.g., fire-fighting. A hand-held or chip-based product that could identify the chemicals given off by fires could be used by fire fighters to indicate the particular fire retardants that would work best and the protective gear to wear, etc. Such `smart` environmental profilers would be suitable for integration into standard room monitoring devices, e.g., thermostats, smoke detectors, etc., and thereby represent a major market opportunity.
Surface acoustic wave resonators are extremely sensitive to both mass and acoustic impedance changes of the coatings in array elements, but the signal transduction mechanism involves somewhat complicated electronics, requiring frequency measurement to 1 Hz while sustaining a 100 MHz Rayleigh wave in the crystal [Grate and Abraham, Sens. Act. B 3:85-111 (1991); Grate et al. Anal. Chem. 65:1868-1881 (1993)]. Attempts have been made to construct sensors with conducting polymer elements that have been grown electrochemically through nominally identical polymer films and coatings [Pearce et al., Analyst 118:371-377 (1993); Shurmer et al., Sens. Act. B 4:29-33 (1991); Topart and Josowicz, J. Phys. Chem. 96:7824-7830 (1992); Charlesworth et al., J. Phys. Chem. 97:5418-5423 (1993)].
Surface acoustic wave resonators are considered problematic in that there is a need to precisely control temperature on each sensor, e.g., to 0.01.degree. C. Also, the sensors generally cannot be manufactured in large numbers, and the complicated technology and complicated circuitry do not allow for chip based sensors.
Electron capture chemical detectors are another example of a well-known chemical sensor technology. The technology is based upon a chemical principle called electron capture, in which electrons are emitted from a filament and captured only by molecules that readily add an electron. These molecules, having added an electron, become negatively charged and can be diverted towards a charged electric plate where they can be detected. A limited number of molecules exhibit this characteristic. These include primarily freons and are the basis for hand-held refrigerant detectors. Freons are responsible for ozone depletion and are considered an environmental hazard. A small chip-based sensor could be useful for detecting freon leaks and could be mounted on refrigeration units for early detection of freon leaks, thus providing near instantaneous detection and mitigation of environmental damage.
Current laboratory-based technology for fluid detection and identification relies heavily on gas chromatography (GC) and mass spectroscopy (MS). These technologies, while important to research and process control and quality control, have their limitations beyond the laboratory setting. Both are confined to bench-top lab analysis and are not suitable for product development outside of research and process control and quality control markets. Often in routine use of GC/MS, one must calibrate the instrumentation based upon what is likely to be in the sample being analyzed. The chemical analysis must be based on a reasonable idea about what is already there, to have confidence in the result of the analysis. A technology is needed that allows odors to be detected in the field, in the office, in the home, or in the industry, in real time. Also, a device is needed that can determine (for a given environment) that unknowns are present requiring further analyses. This is critical in off-odor analysis, such as in quality control, environmental monitoring, and a variety of other applications.
It is therefore an object of the invention to provide a broadly responsive analyte detection sensor array based on a variety of "chemiresistor" elements. Such elements should be simply prepared and are readily modified chemically to respond to a broad range of analytes. In addition, these sensors should yield a rapid, low-power, dc electrical signal in response to the fluid of interest, and their signals should be readily integrated with software- or hardware-based neural networks for purposes of analyte identification.
Further, it is an object of the invention to provide an analyte detection system comprising the sensor, an information storage and processing device, and a fluid delivery appliance. The fluid delivery appliance should be operatively associated with the sensor for delivery of the analyte. No known products, technologies or approaches are capable of generating hand-held general odor detectors. Nor are any known broadly-responsive odor sensors available for many different sensing tasks from the same technology base.
A further object of the invention is to provide a chip-based odor detection system. At present, no known competitive technologies or approaches to odor detection are chip-based. Integration of the sensor, the information storage and processing device, and the fluid delivery system onto a single substrate would provide a significant advancement over existing technology.
II. Relevant Literature
Pearce et al. (1993) Analyst 118:371-377 and Gardner et al. (1994) Sensors and Actuators B 18-19:240-243 describe polypyrrole-based sensor arrays for monitoring beer flavor. Shurmer (1990) U.S. Pat. No. 4,907,441, describes general sensor arrays with particular electrical circuitry.